In the context of Modeling and Model-Driven Engineering (MDE), concrete syntax refers to the specific way a model is visually or textually represented, providing the elements and notations that users interact with when creating or interpreting the model. Concrete syntax defines the symbols, shapes, colors, or textual notations that give a model its tangible form, enabling users to understand and manipulate the underlying abstract concepts (often defined by the metamodel or abstract syntax) in a clear, structured way.

Key Aspects of Concrete Syntax in MDE

1. Visual Representation: In visual modeling languages (like UML), concrete syntax includes symbols, lines, shapes, colors, and spatial arrangement used to depict elements such as classes, relationships, states, or actions. For example, in a UML class diagram, classes are represented by rectangles, and relationships by connecting lines with specific arrowheads indicating association types.
2. Textual Representation: For textual modeling languages or Domain-Specific Languages (DSLs), concrete syntax involves keywords, indentation, delimiters, and operators that define the language structure. For example, in a DSL for configuring workflows, concrete syntax might include specific keywords like “task,” “transition,” and “trigger.”
3. Mapping to Abstract Syntax: Concrete syntax provides a representation of the underlying abstract syntax (the structural and semantic rules governing the model) by giving users a way to see and manipulate these rules. It bridges the modeler’s experience with the system’s formal structure, allowing changes to concrete elements that map back to abstract model properties.
4. Usability and Interpretation: Well-designed concrete syntax improves usability by ensuring that the model is easy to read, interpret, and modify. It leverages familiar symbols or language structures that resonate with the model’s intended users, whether through standard notations (e.g., UML symbols) or custom, domain-specific symbols that simplify understanding.

Concrete syntax is essential in MDE because it:

• Defines the User Interface: It is what modelers directly interact with, providing an intuitive representation of abstract concepts.
• Supports Domain-Specific Customization: By adapting concrete syntax to a specific domain’s conventions, modelers can create representations that align closely with domain experts’ needs and language.
• Facilitates Model Interpretation and Communication: Clear and consistent syntax enhances collaboration by making models more accessible to stakeholders with varying levels of technical expertise.

The concrete syntax can be topological or positional/geometric. 

In summary, concrete syntax in MDE is the tangible layer that transforms abstract modeling concepts into a structured, usable representation, bridging the gap between the formal underpinnings of a model and the practical needs of users.